The invention relates to a method of manufacturing a tub section, in particular rear section, for a washing machine, to a first pre-product and a second pre-product of a tub section, and to a tub section, in particular a rear tub section, for a washing machine.
Nowadays, most tubs of domestic washing machines are made of plastic material(s) except for a ball bearing that is made of metal.
WO 2007/115894 A1 describes a tub of that generic kind for a clothes washing-machine, in particular of the domestic type which houses a rotating drum within which the washing load is placed, said washing tub consisting of a peripheral cylindrical housing formed by a tub front section and a tub rear section which are joined at their respective plastic material edges, enclosing the drum in their interior, by the fusion welding of the plastic materials of said edges, forming a weld bead between the tub front section and the tub rear section where the edges are joined by at least two joining lines.
WO 2007/115904 A1 discloses a tub for a clothes washing-machine, in particular of the domestic type which houses a rotating drum within which the washing load is placed, said washing tub consisting of a peripheral cylindrical housing formed by a tub front section and a tub rear section which are joined, enclosing the drum in their interior, by plastic fusion welding, where the tub front section features at least one front lug and the rear tub section features at least one rear lug, which face each other on closing the tub and to which a damper is fixed by means of a pin, where the pin is formed of a single piece on one of the lugs and is joined to the other lug by means of plastic fusion welding.
Commonly, a plastic compound is used that comprises a polypropylene (PP) base loaded with a filler such as talc or fibre glass. Usually, talc is used as reinforcement filler material in washing machines of low revolutions and fibre glass in those of high revolutions.
Since the tub holds the revolving drum of the washing machine, the tub usually comprises two tub sections, namely a front section and a rear section, that are joined after the drum has been inserted. The rear tub section usually is the one that has greater requirements regarding strength because it holds the bearing housing that in turn is part of a ball bearing leading a driveshaft to the drum in order to spin it. Front and rear tubs are at the moment injected in one go at two different cavities engraved within the same mould.
During the manufacturing process, the most common technique is to inject said plastic compound (PP+filler load) via the bearing holding; this holding (or ‘hub’) itself acts as a core for the whole set, and a metal-plastic union takes place at the interface of both materials. In order to get a proper fastening between the injected plastic compound and the metallic bearing housing, the surface of the bearing housing is provided with numerous grooves to get a fair adherence. On the other hand, and in order to avoid a thermal shock when injection of the plastic is carried out at the metallic bearing housing, the latter must be previously warmed up to a temperature ranging from 40° C. to 60° C. before it is positioned within the mould. Since the bearing housing is made of metal, usually of steel, it has a size and weight directly related to the revolutions with which the drum must spin. Thus, for high revolutions the bearing housing has to be larger that for low revolutions, and its weight changes from one kilogram for the smallest ones to two kilograms for the biggest ones. This amount of warmed up steel in addition to approx. Six liters of PP at 240° C. having a geometry full of regions of great thickness creates some intensely heated zones.
This manufacturing process incurs undesirable consequences. Some of those drawbacks are a not always satisfactory dimensional stability because of a difference of the material contraction rates, cooling curves and different mass ration between metal and plastic. In particular, micro spaces could occur between the plastic and the metal causing a bad adherence. Further, a cooling time of longer than 8 hours that badly affects the whole process (the steel acts the same way as a hot core that keeps heat for a long time like it was a thermos flask). In addition, and due to the long cooling time, it becomes necessary to stock the freshly injected tubs for some hours, which leads to a cost increase.